The present invention relates to a voltage supply control system for a mass filter or analyser, preferably a quadrupole mass filter.
Mass spectrometers that utilise quadrupole mass filters or mass analysers need to apply a high frequency or RF sinusoidal voltage to the rods that comprise the mass filter or mass analyser. For mass spectrometers designed to be able to analyse ions having masses above a few hundred Daltons, a large amplitude RF voltage needs to be applied to the rods. The amplitude of the applied RF voltage may, for example, be several thousand volts.
To avoid excessive power requirements of the drive circuitry it is known to make the load (which includes the quadrupole rod set) resonant at the drive frequency. To ensure mass stability the frequency of the drive is held constant and the amplitude of the drive signal is varied in order to select the mass to charge ratio of interest.
According to a known arrangement the drive frequency is fixed and a variable inductor is manually adjusted during assembly or servicing of the mass filter or mass analyser in order to tune the load so that it is resonant at the drive frequency.
However, the known arrangement suffers from the problem that it is necessary to provide variable inductors which are relatively large and expensive due to the large voltages that they must cope with.
Furthermore, the variable inductors which are used are also large and expensive due to the low losses required from them, otherwise the variable inductors would become excessively hot and power amplifiers associated with the variable inductors would have to supply extra power.
A yet further problem with the known arrangement is that a skilled engineer is required in order to manually adjust the variable inductors so that the load is resonant at the drive frequency.
U.S. Pat. No. 7,973,277 (Rafferty) discloses an RF drive system for a mass filter. The drive system has a programmable RF frequency source coupled to an RF gain stage. The RF gain stage is transformer coupled to a tank circuit formed with the mass filter. The power of the RF gain stage driving the mass filter is measured using a sensing circuit and a power circuit. A feedback value is generated by the power circuit which is used to adjust the RF frequency source. The frequency of the RF frequency source is adjusted until the power of the RF gain stage is at a minimum level. The frequency value setting the minimum power is used to operate the RF drive system at the resonance frequency of the tank circuit formed with the transformer secondary inductance and the mass filter capacitance.
US 2012/0286585 (Thomsen) discloses a high frequency voltage supply system for supplying a multipole mass spectrometer with a high frequency AC voltage which is used to generate a multipole field.
It is desired to provide an improved voltage supply system for supplying an RF voltage to an ion-optical component and a method of supplying an RF voltage to an ion-optical component of a mass spectrometer.